1. Field of the Invention
The present invention relates to anti-oxidant containing polyolefin compositions resistant to yellowing and including a pearlescent pigment.
2. Description of the Prior Art
Many pearlescent or nacreous pigments are based on micaceous substrates which have been coated with a metal oxide layer. As a result of reflection and refraction of light, these pigments exhibit pearl-like luster and depending on the thickness of the metal oxide layer, they can also exhibit interference color effects.
Commercially, the pearlescent pigments encountered most often are the titanium dioxide-coated mica pearlescent pigments. However, such pigments can cause yellowing of anti-oxidant containing polyolefin compositions in which they are contained.
U.S. Pat. No. 4,209,430 relates to a process for suppressing yellowing in thermoplastic polyolefins containing a phenolic anti-oxidant and TiO.sub.2 pigment by adding a phosphorylated polyene either directly to the polyolefin or as a coating on the pigment.
U.S. Pat. No. 4,357,170 reviews various solutions to the problem of yellowing of titanium dioxide pigments in polymeric compositions such as polyethylene in the presence of additives such as phenolic antioxidants.
U.S. Pat. No. 5,376,698 describes anti-oxidant containing polyolefin compositions including titanium dioxide-coated mica particles having a coating of silica and alumina calcined thereon to prevent yellowing of the polyolefin composition.
U.S. Pat. No. 4,435,220 teaches coating platelets with a colored metal oxide or hydroxide, optionally commingled with a colorless oxide or hydroxide, which is improved by including a small amount of an alkaline earth metal in the reagent. The process is stated to provide for transparent colored pigments with improved dispersability, better gloss, clearer color and improved stability to heat and weathering. Improved anti-yellowing properties in the presence of an anti-oxidant is not disclosed.
U.S. Pat. No. 5,271,770 teaches a pigment in which the surfaces of a platelet-like substrate is coated with calcium sulphate and then covered with titanium or zirconium oxide. The resulting pigment is indicated to possess good adhesion and spreadability and has the effect of reflecting ultraviolet rays and infrared rays. No mention is raised about the possibility of using this pigment in a polyolefin composition containing an anti-oxidant.
In addition to the yellowing problem, there is also a problem of photodarkening. This can occur when the pigment is exposed to light, whether in the form of a loose powder or incorporated in a matrix such as an automotive paint, cosmetic formulation, plastic formulation, ink formulation, etc. When photodarkening occurs, the appearance or performance of the pigment, or both, becomes reduced.
There remains a need for an anti-oxidant containing polyolefin composition containing a pearlescent pigment which resists photodarkening and/or yellowing of the polyolefin composition. It is accordingly the object of the present invention to provide such a composition as well as providing a pearlescent pigment for use in such composition and a method of preventing photodarkening and/or yellowing of the anti-oxidant containing polyolefin composition.
These and other objects of the invention will become apparent to those skilled in this art from the following detailed description.